The alleged assault of a Hutt school principal happened after a parent raised concerns about a convicted paedophile visiting the school.

St Joseph's School principal Peter Ahern was allegedly assaulted in his office by a 39-year-old man, a pupil's uncle, who will face assault charges this week in the Upper Hutt District Court sitting in Lower Hutt.

The incident - last Wednesday morning - followed a meeting between Ahern and two relatives of students, who were concerned about a man who visited the school.

The parents had learned the man, in his 60s, was convicted of paedophile offences more than 20 years ago.

Ahern needed stitches for a split head and had bruised ribs after the incident, likened to a 'home invasion" by the school's board of trustees chairman Mark Petersen.

The meeting had been amicable until, at its conclusion, the assault happened, Mr Ahern alleged.

"They were looking for the best interests of their child ... [then] one of the guys lost the plot.''

Mr Ahern confirmed the pair raised the issue of the man being on the school grounds and visiting a classroom.

The man had come to the school at the end of the day when there were a lot of parents about, he said.

''I had no idea that he was on site and I had no idea that he had a conviction,'' Mr Ahern told The Upper Hutt Leader.

''I made it clear to them that this person was not in any way employed by the school.

''He will not be welcome [at the school in the future],'' Mr Ahern said.

The man lives in Upper Hutt and is a long-time member of the St Joseph's Church, adjacent to the Catholic primary school in Pine Ave.

Bill Kirkup, from the church's pastoral committee, confirmed the visit by the man to the school and that he had also been there on an earlier occasion.

"He's a very active parishioner and he went to the school to see a teacher, a director of religious studies."

Mr Kirkup said the man was part of a "parish group presentation" delivering a religious programme to students at the church.

"He was supervised at all times," Mr Kirkup said.

Mr Kirkup said he learnt the man was part of the parish programme for the school after the assault.

"If I had known he was part of the group I wouldn't have let him be part of the presentation [to the students]."

Mr Kirkup spoke to the Upper Hutt Leader after comment was sought from the parish priest Father Ron Bennett, who is also a member of the school board of trustees.

''Father Bennett was unaware at the time that [the man] was in this religious programme,'' Mr Kirkup said.